Ventilation of the lungs is essential to life. Patients in need of medical attention often require assistance with breathing as a result of injury, trauma, or airway obstruction. In an apneic patient, the lungs must be artificially ventilated to ensure proper oxygenation and exchange of gasses within the body. Endotracheal intubation is the technique of placing an endotracheal tube (ETT) into the trachea of a patient, for the purposes of establishing an airway. It is estimated that over 18 million intubations are preformed each year in the US, the majority in the operating room. Because of the close proximity of the esophagous to the trachea, an intubationist may accidentally place the ETT into an esophagous. If the error is detected immediately, no harm results. However, if the incorrect placement is not recognized and the ETT repositioned, within a few minutes irreversible brain damage and/or death may result, due to the lack of oxygen. In an ideal setting, an intubationist places an ETT in one hand and visualizes the glottic opening of the airway by introducing a larygoscope into the mouth. The ETT is then carefully passed into the trachea and attached to a source of oxygen. However, even under such controlled conditions inadvertant esophageal intubations have been reported. (White, S. J. and C. M. Slovis, Acad Emerg Med 1997:4:89–91) This problem is magnified in out-of-hospital intubations performed, or in the hospital outside of the OR and ER wards and intensive care units under less than ideal conditions such as, for example, at the scene of an accident, or within an ambulance in route to a hospital. Under such stressful and often chaotic conditions, misplacement of an ETT in a patient is not uncommon.
In an effort eliminate the danger inherent in unrecognized esophageal intubation, numerous clinical methods and devices have been developed to rapidly assess tube placement. Qualitative methods include direct visualization, observation of chest movement with bag inflation, auscultation of breathing sounds, absence of epigastric sound with ventilation, presence of exhaled tidal volume, reservoir bag compliance, endotracheal cuff maneuvers, absence of air escape, tube condensation with exhalation, absence of gastric contents within the tube and others (O'Connor, R. E. R. A. Swor Prehospital Emergency Care July–Sept. 1999). Each of these methods have utility in a limited range of clinical conditions.
Quantitative methods have also been employed to better assess endotracheal tube placement. The most common quantitative means to document correct placement of an endotracheal tube (ETT) is to sense carbon dioxide during the exhalation phase of ventilation. The success of this method is based on the difference between the CO2 concentration in exhaled air (5%) and the CO2 concentration in esophagael gas (0.2%–0.3%). Documentation of carbon dioxide in the exhaled breath has become the accepted standard for verifying the correct placement of an endotracheal tube, unless the location can be directly visualized, for instance, with a fiberoptic bronchoscope. Numerous devices have been developed for assessing proper endotracheal tube placement through detection of CO2, the utility of each varying with the particular clinical condition.
The most common CO2 detection device employed in hospitals is the capnometer. This device is used to monitor the concentration of exhaled carbon dioxide in order to assess the physiologic status of a patient. The device comprises an infrared sensor that continuously monitors and displays CO2 concentration and generates a waveform (capnogram) that is correlated with a patient's respiratory cycle to quantitatively assess the adequacy of ventilation.
However, these devices are less reliable when there is a pulmonary embolis or a patient is in cardiac arrest. (Garnett, A R et al., JAMA 1987; 257: 512–515) Further, traditional capnometers are expensive, sophisticated, and fragile instruments requiring careful calibration and a source of power, making their use in out-of hospital procedures impractical. Thus, in emergency-type settings, a capnometer may be inadequate. In those situations, alternative devices are employed. The most reliable method for verifying proper tube placement in out-of hospital applications is through use of an end-tidal carbon dioxide detection device (Ornato, J P Ann. Emerg Med 1992; 21:518–523) The EasyCap End-Tidal CO2 Detector (Nellcor-Mallincrodt-Tyco) is an inexpensive, disposable device that quickly attaches to an ETT to sense exhaled CO2. With each breath, CO2 exhaled passes over an indicator in the device that has been treated with a chemical that turns color in response to high CO2 concentration. Thus, a change in color of the indicator is indicative of proper placement of the ETT in the trachea because of the substantially higher concentration of CO2 in exhaled air as previously discussed. Tube placement anywhere but the trachea will not yield a color change. However, even if the ETT is properly placed, the EasyCap, like the more expensive capnometer, is inadequate in those instances where a patient lacks a pulse or has very poor pulmonary perfusion because without CO2 exchange from pulmonary arterial blood to alveoli, insufficient CO2 will be exhaled to produce a color change.
To overcome these problems, other devices that do not directly depend on the detection of CO2 have been developed. These esophageal detector devices (EDD), work on principles of gas volume displacement and depend on the structural difference between the trachea and the esophagus. In one embodiment, a catheter-tip syringe is connected to an ETT via a length of rubber tubing (Wee, M Y K Anaesthesia 1988 43:27–29) In use, negative pressure is created within the ETT through aspiration of the syringe chamber, i.e. withdrawal of the plunger from the syringe. If the ETT is placed in the esophagus, the walls of the esophagus collapse upon the ETT in response to this negative pressure, which in turn restricts air-flow that can be easily detected as resistance to plunger movement. However, when the ETT is placed in the trachea this negative pressure is incapable of causing the more rigid trachea to collapse, thereby allowing free exchange of air. Thus, free movement of the plunger is indicative of proper placement within the trachea. (O'Leary J. J. Anaesthesia and Intensive Care 1988; 16: 299–301) In an alternate embodiment, a rubber bulb is attached to an ETT. (Nunn, J F. Anaesthesia 1988;43:804) In use, the bulb is compressed prior to attachment to the ETT to create a negative pressure within the ETT. As described above, if the tube is placed within the esophagus air passage becomes restricted as the esophageal walls collapse around the ETT, but air freely flows if the ETT is properly placed in the trachea. Thus, passive re-inflation of the bulb is indicative of tracheal intubation, while the failure of the bulb to re-inflate is indicative of esophageal intubation. Each of these devices is portable, inexpensive, easily assembled and provides faster assessment of ETT position than both the capnometer and EasyCap described above, making them particularly well suited for intubations. performed outside the operating room (e.g. in the recovery room, emergency room, intensive care unit and out in the field). Further, these devices are useful in patients experiencing cardiac arrest because the test results do not depend upon the presence of CO2 in exhaled gas (Haridas, R P Update in Anaesthesia 1997;7:6(1)) However, regurgitation of gastric air, distension of the esophagus with air, or an EDD that is not airtight may cause the bulb to re-inflate, giving a false impression of tracheal intubation when the tube is in fact in the esophagus. (Haridas, R P Update in Anaesthesia 1997; 7:6(3)) Further, although these devices are highly accurate when used in a hospital setting, studies indicate that they are only 50% accurate when used in the field by paramedics (Pelicio, M. Acad Emerg. Med 1997:4563–568)
The National Association of EMS Physicians has recognized that no single technique for assessing ETT tube placement currently used is completely reliable in all circumstances. (O'Connor, R. E. and R. A. Swor Pre-Hospital Emergency Care; July-Sept. 1999) Based on the inherent limitations in each device, it has become apparent to the authors that the optimal method of detecting proper ETT placement, particularly in out-of-hospital settings would incorporate both concepts of CO2 detection and gas volume displacement to increase the accuracy of ETT placement to near 100%.